OL moved above previous leaders Lille, who host Marseille later on Wednesday, on goal difference, with injury-hit Paris Saint-Germain two points behind before their game at Bordeaux. Fourth-placed Monaco, four points off top at the start of the day, visit Strasbourg.
Lyon, who were held by Marseille at the weekend, edged a game of few chances thanks to substitute Aouar's fifth league goal of the season.
Memphis Depay wasted the best opportunity of the opening 45 minutes when his effort on the stroke of half-time was saved by Rennes goalkeeper Alfred Gomis. But Lyon started to look more dangerous after the 58th-minute introductions of Aouar and Tino Kadewere off the bench, with the latter seeing a goal ruled out for offside.